One of Britain’s busiest rail lines is to be electrified in a move that will introduce greener and more reliable services for millions of passengers.

The government is finalising plans to transform the Great Western mainline as part of a drive to reduce carbon dioxide emissions from transport. The programme will involve installing hundreds of miles of electric cables as well as alterations to tunnels, bridges and stations on one of Britain’s oldest rail routes.

An announcement could come as soon as Thursday, although the financing is still being put in place. The Department for Transport (DfT) and Network Rail, owner of Britain’s rail infrastructure, have discussed electrifying the route from London Paddington to Cardiff, taking in Reading and Bristol, as well as the popular commuter route from London to Oxford.

However, the programme is expected to be carried out in phases over the next decade in order to minimise disruption.

Britain lags behind many of its European counterparts in electrical coverage of its rail system, with only 40% of the 20,000-mile network electrified. Lord Adonis, the transport secretary, has pledged to electrify swaths of the network, led by Great Western and the Midland mainline from St Pancras to Sheffield, in order to reduce carbon dioxide emissions from transport by 14% by 2020.

Train operators said electrification would bring quicker and more reliable services for passengers, as well as giving rail a green edge over car and air travel. Michael Roberts, chief executive of the Association of Train Operating Companies, said: “Electrification brings with it the dual benefits of helping to make rail services more attractive to customers and drawing them away from cars and planes. It also relies on lower-carbon sources of energy.” First Great Western, the main operator on the Great Western network, carries 84 million passengers a year.

According to Network Rail, the diesel trains that travel on the Great Western route emit at least double the carbon dioxide output per mile of an electric train. The government-backed company has also calculated that it will cost £800,000 a track mile just to erect the cabling. Once work on tunnels, bridges and culverts is added in electrifying the 118-mile stretch from London to Bristol could cost £380m, according to Network Rail.

It is understood that the DfT and Network Rail have discussed funding the work through an increase in Network Rail’s borrowings. Network Rail’s debt is underwritten by the state and the government will pay off the interest over a number of decades, minimising the immediate impact on the taxpayer.

Stephen Glaister, professor of transport and infrastructure at Imperial College London, said the benefits of electrifying thousands of miles of railway track would be undermined if trains were not powered by energy produced from low-carbon sources such as nuclear plants or wind farms. Otherwise, electrification would simply increase demand for electricity from coal- and gas-powered plants, he added. “The government has to clarify where the electricity is coming from. In a world where nuclear power is declining and renewables cannot fill the gap, where else is it going to come from apart from burning more coal and gas?”

Lord Adonis, the transport secretary, said last week: “Transport accounts for a significant amount of our domestic emissions. Therefore decarbonising this sector has to be front and centre of efforts to meet our obligations and commitments to tackle climate change.”

The government is also encouraging greater production, and acquisition, of electric and hybrid cars as part of its low-carbon policy.